This invention relates to a novel method of freezing fertilized ova, spermatozoa or the like and an apparatus for freezing the same adapted for carrying out the same method.
An artificial fertilization has been recently carried frequently out to contrive the improvements of breed of domestic animal and of growth of the domestic animal. In this case, fertilized ova and spermatozoa have been perserved by freezing the same.
It was heretofore known to fill the fertilized ova and spermatozoa in a buffer solution contained in a tube and to freeze the buffer solution as a method of freezing the fertilized ova and spermatozoa.
A temperature change with respect to time of a pure substance when the substance is cooled under a constant pressure is generally known as a cooling curve. According to this principle, the substance does not always start immediately freezing when the substance reaches its freezing point, but the substance will start generally freezing after the substance is overcooled to the temperature lower than its freezing point. Simultaneously, the substance raises its temperature to its true freezing point, and the substance will then lower its temperature again after the entire substance is completely frozen.
The buffer solution is overcooled at the freezing time in the freezing step according to the abovementioned conventional freezing method which merely cools the substance. Then, the temperature of the substance or buffer solution is thereafter immediately raised. Therefore, the fertilized ova and spermatozoa are dead due to the thermal shock of this abrupt temperature change according to the conventional freezing method.
It has been proposed to avoid such a thermal shock by a method of freezing fertilized ova and spermatozoa to remove the buffer solution cooled to the freezing point and to holding a tube containing the buffer solution with a pincette preserved in liquefied nitrogen, thereby carrying out the freezing from the holding position. This also lacks actual utilization due to the face that the removal of the tube adversely affects the temperature of the buffer solution so that there is a failure in the freezing of the solution and an automatic control is difficult due to the complicated operations.